This invention relates generally to a control circuit for an AC motor and particularly to a latching system for maintaining a two-phase reversible AC induction motor rotating selectively in either direction. While the invention may be employed with any two-phase reversible motor, it is especially attractive when used in conjunction with the motor-drive system in an adjustable motorized hospital bed and will be described in that environment.
A motor operated hospital bed usually includes a hand-held, push button-actuated control device for permitting the patient occupying the bed to remotely control various adjustments of the bed merely by selectively depressing different spring-loaded push buttons. Ordinarily, the mattress-supporting structure of the bed is articulated, being divided into four interconnected sections or panels, namely a back section, a center or seat section, an upper knee or thigh section, and a lower knee or foot section. One motor-driven adjustment that may be controlled by the patient raises or lowers the two knee sections where they join together, thereby controlling the position of the patient's knees. Another adjustment, under the patient's control, pivots or tilts the back section with respect to the center section so that the patient's back and head may be raised or lowered. A third motor-driven adjustment may be controlled by the hand-held, push button-actuated control device to vertically adjust the entire mattress-supporting frame.
On some adjustable hospital beds, a duplicate set of push buttons are mounted on a control panel, called a "nurse station", on the bed's foot board in order that a nurse or attendant can conveniently control the same adjustments or functions that are controllable with the patient's hand-held device. The switches actuated by the patient are of the momentary contact type which requires the patient to maintain continuous pressure on the selected spring-loaded push button until the desired bed position is reached. Preferably, for the most efficient operation the push buttons at the nurse station should not necessitate continuous actuation by the nurse. This is especially preferred for the high-low control that varies the vertical level of the mattress support. Quite often the bed is vertically adjusted by the nurse to either its highest position obtainable or its lowest position obtainable. As an important time-saving feature for the nurse, each of those extreme or limit positions should be reached merely by momentarily actuating a spring-loaded push button. However, these operations should not interfere with the circuitry operated by the patient in order that he may retain the same control and is not locked out.
The desired control for both the nurse and patient is achieved, in accordance with the present invention, by a unique latching system of relatively inexpensive construction. By simply depressing a selected push button-actuated momentary contact switch, the system is latched into either an up operating mode or a down operating mode to cause the high-low motor, which will be of the two-phase reversible type, to run the mattress support to either its high or low limit.